The invention relates to a propeller for commercial, pleasure and racing boats, and more particularly to a base vented propeller for boats driven by large outboard or stern-drive motors.
With the popularity of boating increasing, the number of damaged propellers needing replacement is also increasing. While many propellers are replaced because of structural damages, numerous others are replaced in an attempt to upgrade the drive system of the boat. These replacement propellers are commonly constructed of cast aluminum or stainless steel.
It is generally believed that an injection molded plastic propeller can be produced cheaper than the standard aluminum replacement propeller. While several plastic propellers are presently on the market, current technology provides limited propeller performance in engine power ranges exceeding 20 horsepower.
In the construction of a plastic propeller, difficulty is generally encountered as a result of an incompatibility between the structural requirements and the hydrodynamic performance requirements of the propeller blades. More specifically, in producing a plastic propeller having adequate strength and stiffness, blade thickness becomes hydrodynamically excessive. At the expense of thrust production, and therefore boat speed, engine power is lost overcoming the high drag induced by the blade's thickness.
It is an object of the present invention to provide a inexpensive propeller incorporating increased thickness while maintaining drag at competitive levels.
It is another object of the invention to provide a high performance, low drag, subcavitating propeller assembly.
The U.S. Patent Application entitled "BASE VENTED SUBCAVITATING HYDROFOIL SECTION", filed Apr. 4, 1990, Ser. No. 509,952, Notice of Allowance dated Apr. 30, 1991, U.S. Pat. No. 5,046,444 commonly assigned to by the Applicant of the present invention, discloses a hydrofoil section that is capable of subcavitating at high speeds while maintaining hydrofoil section drag at acceptable levels. The above application is herein incorporated by reference.
In the present invention, an increased blade thickness is incorporated into each propeller. However, even with the thickness increase, the total section drag of each blade is held at low levels through base venting, as disclosed in the above mentioned application. Each propeller blade has a forward fin portion and a rearwardly flared base or post portion. The fin and post are integrally formed with each other and also with a propeller hub of the standard "flow through" exhaust gas variety. Cavitation is prevented by drawing exhaust gas, exiting the hub, along the rear surface, or trailing edge, of the base and venting a low pressure region developing downstream of each blade. The base portions of each blade are locally flared so as to build high pressure fences along the trailing edge. The high pressure fences prevent ventilation gas (exhaust gas) from flooding into the low pressure region developing on the suction surfaces of the blades and causing back ventilation. Additionally, the flares of the base portions reduce fluid flow velocities over the blade surfaces and in this manner the development of cavitation is delayed. Thus, each blade of the propeller exhibits a drag reduction through base ventilation.
In the present invention, two embodiments of the base vented propeller are disclosed. The first embodiment is constructed of stainless steel and the second of plastic. The blades of the base vented steel propeller have a midchord thickness generally comparable to propellers now present in the industry. However, the increased flaring of the blades and venting of the bases, along with the low drag associated with the subsequent postponement of cavitation, permit the subcavitating performance range of the base vented steel propeller to be extended beyond the abilities of conventional propellers.
By virtue of the thickened base portion, both embodiments of the present invention exhibit an increase in strength relative to propeller blades of the same length and midchord thickness. However, being constructed of plastic, the second embodiment has need for a further strength increase. This is accomplished by increasing the midchord thickness of each blade. Even though the additional midchord thickness also increases the pressure drag of the blade, the strength and subcavitating benefits of base venting allow the plastic propeller of the present invention to exhibit performance specifications corresponding to the aluminum replacement propellers presently on the market. While the performance is comparable, the base vented plastic propeller has substantial advantages, namely, durability and low cost.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.